How to create half circular-window hole on the wall without resulting ngon?
In answer to the question:


Select the face that you want to put an arch opening in
Subdivide enough times to suit your needs:





Select the edges that surround the area where your arch opening will be:





Select the Circle tool from the LoopTools addon (available in User Preferences if not already installed). This will move the selected edges into a circular shape:





At this point, you can either delete the bottom half of the original face and rebuild the mesh for the lower part of your door opening, or you can realign the vertices that will form the door opening by copy/pasting the appropriate transform values to line them up on the z axis. To do this, just place the mouse cursor over the appropriate transform value and press Ctrl-C: 





Individually select each vertex below the one you just copied the value from, and paste the value into the same transform field by just placing the mouse over it and pressing Ctrl-V. You should end up with something like this:





Now you can delete the faces that form the arched doorway.